It is usually desirable and sometimes important to minimize the intrusion of water through a louver. In such instances so-called drainable louvers are often used. The principal characteristic of drainable louvers is the provision of a drainage trough at the lower front edge of each blade for catching water that impinges on the blade and preventing it from dripping off the front edge of the blade and becoming entrained in the air flow or falling onto the blade below and causing a splash of small droplets, some of which will become entrained in the air flow. The drainage troughs open at one or both ends into a drainage channel in a vertical side member--a jamb or a mullion--of the louver frame. Examples of drainable louvers are described and shown in U.S. Pat. Nos. 3,287,870 (Johnson, 1966), 3,782,050 (Dowdell et al., 1974) and 4,103,468 (Olsen, 1978) and British Patent No. 341,556 (White, 1931). In the louver of the Dowdell patent, each blade has a hole in the bottom of each drainage trough that lies behind a front flange of the jamb, and the corner areas of the jambs adjacent the vertically aligned holes in the troughs provides a form of drainage channel, in that water drained through the holes tends to cling to the jamb and run to the bottom of the louver. In the louvers of the Johnson and Olsen patents the drainage channels are U-shaped flange portions of the jambs that open laterally toward the ends of the drainage channels. The British patent proposes vertical downtakes formed by channel members set into notches in the lower front portions of the blades with holes opening laterally to the portions of the drainage troughs on either side and closed at the front by plates.
In many louver installations the environment makes the drainage troughs and channels prone to becoming clogged with leaves and other debris. Leaves and other wind-blown objects fall on the blades, wash into and along the troughs and get caught in the channel, plugging it. The blade immediately above a plug in a channel then becomes the recipient of all water drained from the blades above, which will probably back up into the drainage channel of the blade above the plug. That water overflows the lower front end of the blade and is highly subject to becoming entrained in the air flow entering the louver. Unlike raindrops falling at high velocity in front of the louver, water that falls off the front of the blades has a low velocity--hence the much greater tendency for it to become entrained in the flow and penetrate the louver. Also, water falling from the blade above a plugged drainage channel onto the next lower blade increases the splash and the quantities of small droplets, which are very prone to becoming entrained